This invention relates to magnetometers, and more particularly to magnetometers of the fiber optic type.
Sensing of magnetic fields using fiber optic interferometers has been demonstrated recently. These devices may be used to detect the presence of objects that produce DC magnetic fields. In these devices, a two-arm Mach-Zehnder fiber interferometer is used wherein one of the fiber optic interferometer arms serves as a sensor arm on which some magnetostrictive material is deposited. When exposed to a magnetic field, the magnetostrictive material will stretch the sensor fiber while the reference fiber remains unaffected. As a result, a magnetically induced differential path length change or phase shift is introduced at the output of the interferometer.
In practice, all ferromagnetic materials exhibit hysteresis behavior to some extent, implying that the performance of the material depends on the magnetic history that the sample has been subjected to. In other words, once the magnetostrictive sensor is exposed to an external magnetic field, removal of the magnetic field will leave a residual material magnetization or sensor signal whose strength depends on the last largest magnetic field applied to the magnetic sample. As a result, any subsequent sensor output is a measure of the applied external magnetic field plus the residual magnetization of the magnetic material. Since the residual magnetization due to magnetic hysteresis is magnetic history dependent, the applied magnetic field measurement is undetermined or ambiguous without knowing the magnetic history or hysteresis of the magnetic material. In order to determine the true strength of an external magnetic field without ambiguity introduced by any material hysteretic effect, some corrective measure is needed.